Relay system



Dec. 5, 1967 1 cH LL ET AL 3,356,908

RELAY SYSTEM I Filed Nov. 6, 1959 RELAY RC. THERM/STOI? 5 I LINE j I I I I I I I l2 ILRELAY /5 H II /IVE Ji I I /3 l I F/G 2 I I I I I l /R w/va L. M/TCHELL I SIDNEY S PRESSMA/V INVENTORS ATTORNEYS 7'0 LOAD United States Patent 3,356,908 RELAY SYSTEM Irving L. Mitchell, Rockville Centre, and Sidney S. Pressman, Brooklyn, N.Y., assignors to Ebert Electronics Corporation, Queens Village, N.Y., a corporation of New York Filed Nov. 6, 1959, Ser. No. 851,352 17 Claims. (Cl. 317-124) The present invention relates generally to relay circuits and more particularly to series resonant relay circuits which require no unilateral control devices for their operation.

Many applications exist for relay controls, wherein current to a load is controlled by a relay which changes its operative state in response to an activating element which senses a physical magnitude and which changes impedance as a consequence of such sensing. One typical application of this type relates to the operation of relays in response to changes of light intensity, in which case the activating element may be a photo-electric cell, or more specifically a photo-electric cell of the photo-conductive type. Alternative systems involve activating elements which are sensitive to temperature and which change resistance upon change in temperature. More generally, the activating element may be an impedance which is controlled by the value of a physical quantity.

The present invention concerns itself primarily with activating elements of the photo-conductive type, although in its broadest aspect the invention is not so limited but may be utilized in conjunction with any type of variable impedance as an activating element. The invention may be generalized by utilizing an inductive element as the activating element, or a capacitive element as the activating element. Capacitive elements are known which are light sensitive and inductive elements of the saturating type are known which change their impedance or reactance upon change of control current applied thereto.

Activating elements usually are of a type which cannot carry heavy currents, or, in the alternative, are of types which can be manufactured much more economically if made to have extremely small current carrying capacity. For example, a photo-conductor element is not normally capable of carrying heavy currents when fabricated in convenient size, and a saturable inductance may be reduced in size as the current required to be controlled by it it reduced. It is a feature of the present invention to provide a control circuit for a relay, control, being effected in response to an activating element which is not required to draw heavy current or to be of considerable size, and in which the relay is controlled directly by the activating elements without requiring the interposition of an amplifyirig device, such as a vacuum tube, a transistor, or the like. Thereby, the cost of a relay system may be radically reduced, and its reliability under long term operating conditions may be radically increased.

Briefly describing a preferred embodiment of the invention, a relay coil is connected in series With a condenser, with which it resonates or approximately resonates at the operating frequency of the system. The present system is designed and intended primarily for energization from power lines, i.e., at 60 cps. The activating element, which in the preferred embodiment of the invention isa photo-conductive cell, is connected preferably across the relay coil. In the absence of light the photo-conductive element has an extremely high resistance, and accordingly does not materially affect the series circuit consisting of the tuning condenser and the relay coil. However, when the photo-conductive element is illuminated, the Q of the series circuit is reduced, and the reduction 3,356,908 Patented Dec. 5, 1967 may be radical. Accordingly, the total voltage across the relay coil, which is higher than line voltage under resonant or near resonant condition, may be reduced by reduction of ,the Q of the circuit to a comparatively low value, so that the relay coil transfers from an operating to a nonoperating condition. Effect of the photo-conductive cell on the Q of the circuit is enhanced so far as current drawn by the relay coil is concerned, by the fact that the photoconductive element shunts the relay coil and that the shunting effect is relatively slight when the photo-conductive cell is unilluminated, but becomes considerable when the photo-conductive cell is illuminated. Nevertheless, the photo-conductive cell is not required to pass relay current, so that a heavy relay may be operated by means of the present circuit in response to a photo-conductive cell which is small and of low cost. Furthermore, the life of the photo-conductive cell is lengthened by the fact that the photo-conductive cell is not required to carry heavy currents. As an alternative circuit, the photo-conductive element may be connected across the tuning condenser instead of across the relay coil, in which case illumination of the photo-conductive element changes the Q of the resonant circuit and thereby is enabled to control operation of the relay. However, the total effect in the latter case is smaller than when the photo-conductive element is connected directly across the relay, because the shunting eifect is lost.

It is accordingly a broad object of the present invention to provide a relay system, in which the relay coil is contained in a series resonant circuit, and in which a variable impedance is associated with the series resonant circuit in such fashion as to reduce the Q value of the resonant circuit, upon change of value of the impedance element, sufficiently to change the operating state of the relay.

It is a more specific object of the present invention to provide a relay circuit in which the relay coil is connected in series with a tuning condenser, to form a series resonant circuit, and in which a variable resistance is associated with the resonant circuit in such fashion as to reduce the Q of the resonant circuit upon reduction of resistance of the resistive element.

A further object of the invention resides in the provision of a relay system including a relay coil and a tuning condenser in series with the relay coil and resonating therewith, in which a variable impedance is connected across the relay coil alone, the variable impedance having values varying between very high values which do not effect the Q of the resonant circuit, and relatively low values which reduce the Q of the resonant circuit sufficiently to vary the operative state of the relay.

Still another object of the present invention is to provide a relay system employing a relay coil in series with a tuning condenser which resonates with the inductance of the relay coil, the relay coil being shunted by a photoconductive element which controls the current flow in the relay coil as a function of light intensity impinging 011 the photo-conductive element.

In accordance with a modification of the present inven tion a thermistor is connected in series with the activating element across the relay coil of a relay system employing a relay coil connected in series with a tuning condenser to provide a series resonant circuit. The function of the thermistor in the last described embodiment of the present invention is to provide a resistance which is relatively high, while the photo-conductive cell is unilluminated, but which becomes relatively low when the photo-conductive element becomes illuminated and passes current through the thermistor. The thermistor provides a delay element for the system, Which cannot change resistance rapidly, so that, when the relay is in energized state, should there be a transient increase of light on the photo-conductive element the total resistance in parallel with the relay coil will remain sufiiciently high to maintain the relay in energized state.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawing, wherein:

FIGURE 1 of the drawing is a schematic circuit diagram of a relay circuit in accordance with an embodiment of the present invention; and

FIGURE 2 is a schematic circuit diagram of a modification of the system of FIGURE 1.

Referring now more specifically to the accompanying drawing, the reference numeral denotes a relay coil, associated with relay contacts 11, which are in series with terminals 12 in series with which, and with a suitable power source, may be connected any desired type of load, such as lamps. A source of line voltage 13 is provided, which in a preferred embodiment of the invention may be a 60 c.p.s. power line. The terminals 13 are connected across the coil 10 via a capacitor 14, which has a value such as to resonate with the inductance of the coil 10 at the frequency of the line 13. It follows that the total current flowing to the relay 10 will be greater than would be the case in the absence of the condenser 14, and that the voltage across the coil 10 will be greater than line voltage available at the terminals 13, by a factor equal to the Q of the resonant circuit comprising the coil 10 and the condenser 14. The Q of the circuit is primarily determined by the resistance of the coil 10, and more accurately in terms of the ratio of its resistance to its inductive reactance at the operating frequency. Connected directly across the coil 10 is a photo-conductive cell 15, in series with a thermistor 16. It will then be realized that the relay coil 10, while illustrated as having normally open contacts, may be of the type which has normally closed contacts, and in which energization of the relay coil 10 results in opening of the contacts. When the photo-conductive element 15 is sufiiciently illuminated, the resistance across the coil 10 becomes relatively small. In such case, the phase of the voltage across the condenser 15 is no longer opposite to the phase of the voltage across the coil 10, and moreover the relative magnitudes of the two voltages become unequal, which may be described as reducing the Q of the resonant circuit. The voltage across the coil 10 is reduced by this reduction in Q, and the current in the series resonant circuit is reduced, reducing operating current to the relay coil 10. The relay coil 10 accordingly permits the contacts 11 to open.

The elfect of the thermistor 16, in the circuit, is to introduce a delay. When the photo-conductive element 15 is illuminated current flow through the photo-conductive element and through the thermistor 16 increases. This increase in current flow gradually heats the thermistor and correspondingly radically reduces its resistance. However, should the photo-tube 15 become illuminated for a short instant of time, the relay would not operate, because the thermistor element 16 in series with the photoconductive element 15 does not reduce its resistance until a sufiicient time has elapsed for its temperature to increase by a considerable amount. It follows that if the system is in un-illuminated condition the relay coil 10 cannot be caused to pull up the contacts 11 unless relatively sustained illumination is provided for the phototube 15.

One of the applications of the present invention is to illumination control systems, i.e., to closing the energizing circuit to electric light systems, when ambient conditions are dark, i.e., on approach of night-fall. In this application of the system as illustrated, so long as the photoconductive cell 15 is in sun light, or day light, operating current to the relay coil 10 is reduced sufficiently to maintain the contacts 11 open. However, as night falls the photo-conductive cell 15 becomes subject to light of sufficiently reduced intensity that its resistance becomes very high. In such case, the thermistor 16 is deprived of heating current and its resistance becomes very high. The shunt circuit across the coil 10 then becomes of sufiiciently high total resistance as to become ineifective circuit-wise, and the relay coil 10 then finds itself in a series resonant circuit, in which relatively heavy currents flow. The contacts 11 then close, energizing a source of artificial illumination, i.e., lamps. However, if there should be a lightning storm, for example, or if a passing vehicle with brilliant headlamps should transiently illuminate the photo-cell 15, it is not desired that the relay 10 release its contacts 11. In accordance with the present invention, this will not occur, because any transient illumination of the photo-conductive element 15, while this reduces the resistance of the photo-conductive element 15 after only a very short delay, leaves the thermistor 16 at its high resistance value for a considerable period of time, and unless the photo-conductive cell 15 is at relatively low resistance value for a sufliciently long period of time the thermistor 16 will sustain a sufficiently high value of resistance to prevent operation of the relay.

While we have described the present invention as utilizing a photo-conductive element 15 as an activating element for the system, it will be appreciated that any variable resistance may be substituted for the photo-conductive element 15, exemplary elements being heat sensitive resistances, light sensitive capacitors, current sensitive inductors, and the like. So long as the element substituted for the photo-conductive element 15 can be varied between a very high value of impedance and a very low value of impedance, the circuit comprising the capacitor 14 and the coil 10 can be varied between a resonant value and a non-resonant value, or a value in which the Q of the circuit is radically reduced, and the system will effectively operate. In any of these cases, moreover, the activating element, with or without thermistor 16 in series therewith, constitutes a shunt across the relay coil 10 and accordingly has a control elfect which is cumulative to the control effect which is related to the series resonant circuit as such, so that two cumulative effects tend more positively to control the current through the relay 10 than is the case for either effect alone, assuring action, and assuring that the system will not chatter and will not respond to transient changes of activating effect.

Referring to FIGURE 2 of the drawings, the photoconductive cell 15 and the thermistor 16 are shown connected in shunt to the condenser 14 rather than in shunt to the coil 10. The systems of FIGURES 1 and 2 are otherwise identical, and their operations are similar. The system of FIGURE 1, however, possesses the advantage that the photo-conductive cell 15 not only reduces the Q of the resonant circuit comprising condenser 14 and coil 10, but also by-passes some of the line current around the coil 10, thereby providing a double control elfect.

In one practical embodiment of the invention, the Q of the resonant circuit was 2 at its highest level, so that the voltage across the coil equalled twice line voltage. With the photo-conductive cell 15 illuminated, the voltage across the coil dropped to approximately /3 line voltage. The capacitor 15 had a value of .5 microfarad and the resistance of the relay coil was 2.5 kilohms. For low resistance coils it is advisable to insert a protective resistance in series, to bring the total series resistance to approximately 2.5 K.

While we have described and illustrated one specific embodiment of our invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

What we claim is:

1. A relay system operative from an AC. source of power, comprising a relay coil, operating contacts operable only in response to a predetermined minimum voltage across said relay coil, a capacitive reactance connected in a series circuit with said relay coil and with said A.C. source of power, said relay coil having inductive reactance at least approximately numerically equal to said capacitive reactance at the frequency of said AC source of power, said series circuit having a Q of greater than unity, whereby the voltage across said relay coil is greater than the voltage of said A.C. source of power, and variable impedance means connected in shunt to one of said capacitive reactance and said relay coil for controllably modifying said Q to a value below unity, said last means comprising a variable impedance including a photo-conductive cell.

2. The combination according to claim 1 wherein said variable impedance includes a variable resistance.

' 3. The combination according to claim 1 wherein said variable impedance is connected directly across said relay coil.

4. The combination according to claim l wherein said variable impedance is a photo-conductive cell connected directly across said relay cOil.

5. A relay system operative from an A.C. source of power, comprising, a relay coil, a capacitive reactance connected in a series circuit with said relay coil and said A.C. source of power, said relay coil having inductive reactance at least approximately numerically equal to said capacitive reactance at the frequency of said A.C. source of power, said series circuit having a predetermined relatively high Q greater than unity, means connected in shunt to one of said capacitive reactance and said relay coil, said last means comprising a variable impedance responsive to a signal for reducing its impedance and thereby said Q to a relatively low value less than unity, a pair of contacts operatively associated with said relay CO1 6. The combination according to claim 5 wherein said variable impedance includes a photo-conductive device.

7. The combination according to claim 6 wherein said variable impedance is connected directly across said relay coil.

8. The system according to claim 7 wherein said photocoriductive device is connected directly across said relay cor 9. In a system for operation from an A.C. power source of predetermined frequency, a relay coil having inductance, capacitive means connected in a series circuit with said relay coil and series resonating therewith at said predetermined frequency, said series circuit being connected across said AC. power source, contacts operatively associated with said relay coil, said contacts being arranged to operate in one sense in response to relatively high current in said relay coil and in the opposite sense in response to relatively low current in said relay coil, and a photo-sensitive resistance connected in shunt to said relay coil only.

10. A relay system operative from an alternating current source of power, comprising a relay having a relay coil, a capacitive reactance connected in a series circuit with said relay coil across said source of power, said relay coil having inductive reactance substantially equal to said capacitive reactance at the frequency of said alternating current, said series circuit having a Q value of substantially greater than unity, variable impedance means, means connecting said variable impedance means in shunt to one of said capacitive reactance and said relay coil, said variable impedance means including a resistive component of value sufiicient to vary said Q value from substantially above unity to substantially below unity.

11. The combination according to claim 14) wherein the values of voltage across said coil consequent on variations of said Q values are approximately twice and one third line voltage.

12. A relay system operative from an AC source of power, comprising a relay coil, a capacitive reactance connected in a series circuit with said relay coil across said AC source of power, said relay coil having inductive reactance at least approximately numerically equal to said capacitive reactance at the frequency of said AC source of power, said series circuit having a Q of greater than unity, and impedance means connected in shunt to one of said capacitive reactance and said relay coil for controllably modiflying said Q to a value below unity, said last means comprising a variable impedance includinga photo-conductive cell connected in series with a resistance which inherently reduces in resistance on increase of current therethrough.

13. A relay system operative from an AC source of power, comprising a relay coil, a capacitive reactance connected in a series circuit with said relay coil across said AC source of power, said relay coil having inductive reactance at least approximately numerically equal to said capacitive reactance at the frequency of said AC source of power, said series circuit having a Q of greater than unity. and impedance means connected in shunt to one of said capacitive reactance and said relay coil for controllably modifying said Q to a value below unity, said last means comprising a variable impedance including a photo-conductive cell and wherein said variable impedance is a photo-conductive resistance in series with a thermistor and both connected in shunt only to said relay coil.

14. A relay system operative from an A.C. source of power, comprising a relay coil, a capacitive reactance connected in a series circuit with said relay coil and said A.C. source of power, said relay coil having inductive reactance at least approximately numerically equal to said capacitive reactance at the frequency of said A.C. source of power, said series circuit having a predetermined relatively high Q greater than unity, means connected in shunt to one of said capacitive reactance and said relay coil, said last means comprising a variable impedance responsive to a signal for reducing its impedance and thereby said Q to a relatively low value less than unity, a pair of contacts operatively associated with said relay coil, wherein said variable impedance includes a photo-conductive device, and wherein is further provided a thermistor in series with said photo-conductive cell, said thermistor having a negative temperature coeflicient of resistance.

15. A relay system operative from an A.C. source of power, comprising a relay coil, a capacitive reactance connected in a series circuit with said relay coil and said A.C. source of power, said relay coil having inductive reactance at least approximately numerically equal to said capacitive reactance at the frequency of said A.C. source of power, said series circuit having a predetermined relatively high Q greater than unity, means connected in shunt to one of said capacitive reactance and said relay coil, said last means comprising a variable impedance responsive to a signal for reducing its impedance and thereby said Q to a relatively low value less than unity, a pair of contacts operatively associated with said relay coil, wherein said variable impedance includes a photoconductive device, wherein is further provided a thermistor in series with said photo-conductive cell, said thermistor having a negative temperature coefficient of resistance, and where said variable impedance and said thermistor in series are connected directly across said coil.

16. In a system for operation from an A.C. power source of predetermined frequency, a relay coil having inductance, capacitive IIHCEIDS connected in a series circuit with said relay coil and series resonating therewith at said predetermined frequency, said series circuit being connected across said A.C. power source, contacts operatively associated with said relay coil, said contacts being arranged to operate in one sense in response to relatively high current in said relay coil and in the opposite sense in response to relatively low current in said relay coil,

and a photo-sensitive resistance connected in shunt to said relay coil only, wherein is provided a thermistor having a negative temperature coefficient of resistance connected in series with said photo-conductive resistance in shunt to said relay coil.

17. A relay system operative from an AC source of power, comprising a relay coil operating contacts 0pera'ble only in response to a predetermined minimum voltage across said relay coil, a capacitive reactance connected in a series circuit with said relay coil and with said AC source of power, said relay coil having inductive reactance at least approximately numerically equal to said capacitive reactance at the frequency of said AC source of power, said series circuit having a Q of greater than unity whereby the voltage across said relay coil is greater than the voltage of said AC source of power, and variable impedance means connected in shunt to one of said capacitive reactance and said relay coil for controllably modifying said Q to a value below unity, said last means comprising a variable impedance including a photoconductive cell, a thermistor, said photoconductive cell being connected in series with said thermistor.

Tennan: Radio Engineering,

References Cited UNITED STATES PATENTS Traver 317-147 XR Pell 317-147 X White 317-128 Mead 317-128 X Mitchell et a1. 317-125 XR Howell 307-117 FOREIGN PATENTS Great Britain.

Great Britain.

France.

OTHER REFERENCES 3rd edition, 1947,

McGraw-Hill Book Co., Inc., pp. 39-52. 

1. A RELAY SYSTEM OPERATIVE FROM AN A.C. SOURCE OF POWER, COMPRISING A RELAY COIL, OPERATING CONTACTS OPERABLE ONLY IN RESPONSE TO A PREDETERMINED MINIMUM VOLTAGE ACROSS SAID RELAY COIL, A CAPACITIVE REACTANCE CONNECTED IN A SERIES CIRCUIT WITH SAID RELAY COIL AND WITH SAID A.C. SOURCE OF POWER, SAID RELAY COIL HAVING INDUCTIVE REACTANCE AT LEAST APPROXIMATELY NUMERICALLY EQUAL TO SAID CAPACITIVE REACTANCE AT THE FREQUENCY OF SAID AC SOURCE OF POWER, SAID SERIES CIRCUIT HAVING A Q OF GREATER THAN UNITY, WHEREBY THE VOLTAGE ACROSS SAID RELAY COIL IS GREATER THAN THE VOLTAGE OF SAID A.C. SOURCE OF POWER, AND VARIABLE IMPEDANCE MEANS CONNECTED IN SHUNT TO ONE OF SAID CARPACITIVE REACTANCE AND SAID RELAY COIL FOR CONTROLLABLY MODIFYING SAID Q TO A VALUE BELOW UNITY, SAID LAST MEANS COMPRISING A VARIABLE IMPEDANCE INCLUDING A PHOTO-CONDUCTIVE CELL. 